A polishing apparatus is used for polishing and planarizing a substrate such as a semiconductor wafer. There is known a polishing apparatus having a top ring with multiple chambers whose inner pressures are adjustable independently. In this type of polishing apparatus, a sensor measures a physical quantity associated with a thickness of a film on a substrate, and a monitoring signal is produced based on the physical quantity. Prior to polishing of the substrate, a reference signal that indicates a relationship between the monitoring signal and times is prepared in advance. During polishing of the substrate, pressing forces of the top ring are adjusted such that monitoring signals, obtained at plural measuring points on the substrate, converge on the reference signal, whereby a uniform film thickness can be realized over a surface of the substrate (for example, see WO 2005/123335).
However, in the conventional polishing apparatus, a sensor signal value, obtained at a certain zone of a substrate, may greatly differ from sensor signal values obtained at other zones. This is problematic in evaluating a film thickness correctly by the sensor. One of causes of this problem is signal drop due to an effective measuring range of the sensor. The effective measuring range of the sensor necessarily has a certain dimension. Consequently, when the sensor is measuring a periphery of a wafer, part of the effective measuring range of the sensor protrudes from a surface of the wafer and the sensor cannot obtain accurate signals. In such a case, it is possible to exclude the measuring points where the accurate signals cannot be obtained. However, in a case where the uniformity of the film thickness in the periphery of the wafer is of especial importance, this method cannot be used.
Another cause is an influence of metal or magnetic material in the top ring. If a conductive metal (e.g., SUS) or a magnetic material is used in the top ring, the sensor signal value can be locally changed by the influence of such a material.